The electric power transmission networks of ships and power plants are relatively small. Generators are connected thereto for supplying the ship's demand of electric power. Each generator is driven by a power source, such as a diesel engine. The generators run in the electric network at the same speed so as to have the same frequency in all places of the electric network. It is not possible to economically store electricity, so the production of electricity must correspond to the consumption of electricity of the network. Thus there must be some kind of arrangement in the power transmission network for distributing the load among the generators.
A known way is to use droop adjustment, i.e. deviation adjustment. Each generator has a power production line as a function of the frequency of the network. When the load is large, the frequency of the network tends to increase, and when the load is small, the frequency tends to decrease. Thus, the disadvantage is that the frequency of the constant state of the system depends on the load of the system.
Another way is to use so-called isochronic load distribution. In this method the average load of the system is used for establishing the set values for the generators for distributing the load. The principle of isochronic adjustment is similar to deviation adjustment, but it does not depend so much on the load of the network. In an isochronic load distribution the generators of the network are made to run well so that they produce the same frequency regardless of the load of the network.
Use of digital data communication buses for replacing the older analogue buses used for load distribution among generators has become more common. The advantages of digital buses are their accuracy, reliability and robustness. With a digital solution, it is, however, difficult to mimic an analogue power transmission network. Currently the digital data bus solution is the prevailing solution. Often a CAN bus is used as the data communication bus. Usually isochronic load distribution is used. Each generator of the network updates its process values which are presented in a data vector in its generator-specific part. The data vector is broadcast to other generators of the network. The updated part of the vector contains generator-specific signal and parameter data needed for isochronic load distribution. If one or more generators of the network is unable to send updated data, the load distribution arrangement can not execute load distribution.
The power transmission network comprises switches the state of which can be changed, if necessary, either automatically or manually. If switch is incorrectly open or closed, the generator might send incorrect data for the isochronic load distribution.